1. Field of the Invention
The present invention relates to apparatus for use in a device for coupling light out of a laser ring resonator. More particularly, this invention pertains to apparatus suitable for superposing a pair of light beams that interfere at an angle of divergence to form a pattern of interference fringes at an observation location.
2. Description of the Prior Art
A pair of oppositely directed light waves or beams can propagate in a closed light path in a laser ring resonator ("ring resonator") that includes three or more mirrors. In order to measure the frequency difference (generated by a non-reciprocal effect) between, the two light beams the beams must be coupled out of the resonator. The frequency difference between the light waves can then be measured by the interference pattern between the two beams. Any external effect that generates a frequency displacement between the two light sources can be detected by observation of the interference signal.
The basic structure of a conventional laser ring resonator having four mirrors (A, B, C and D) is illustrated in FIG. 1. The light paths of the (two) oppositely-propagating light waves is indicated by the dashed, closed figure LW. Two mirrors (e.g. the mirrors A and B) are partially-transmissive. A prism I, fitted to the substrate of the mirror B, is equipped, for example, with a pair of light-sensitive detectors G and H. The detectors G and H permit measurement of the intensities of the two propagating light waves, allowing maintenance of a constant light intensity level. The coupling-out of the light to measure the frequency displacement occurs at the partially-transmissive mirror A. A superposition prism E, to which a photodetector F is fitted for scanning the interference pattern resulting from superposition of the oppositely circulating light waves, is coupled to the exterior of the substrate of the mirror A.
An asymmetric prism E is generally utilized to superimpose the beams. Such a prism includes a roof angle that deviates slightly from ninety (90) degrees. As a result, the two beams traverse differing part lengths within the prism. In the event of a change in temperature, this produces phase shifts between the two light waves. As indicated by the dotted lines comprising the light path LW, one beam is deflected by total reflection at the boundary surfaces of the prism E, subjecting it to surface effects. Such effects can influence the phase of the light wave and can disturb the interference signal, causing an apparent frequency difference.